foreach macro for iterating through enum values

Introduction of foreach macro for iterating through enum values. Purpose
of this macro is increase the readability of the code. An example would
be:
	foreach(int *i, SOURCE, DESTINATION)
		...
Merge branch 'next' of https://github.com/alexander-zimmermann/flowgrind into foreach_loop
Unify signal handling
Make MTU, PMTU and IMTU as unsigned int
Move external variables infront of the other global variables
Merge pull request #185 from alexander-zimmermann/next

Nits
Merge pull request #183 from alexander-zimmermann/foreach_loop

foreach macro for iterating through enum values
Merge branch 'next' of https://github.com/alexander-zimmermann/flowgrind into cleanup_pcap